Water supply systems for domestic and commercial use may be high or low pressure water systems, and actual water pressures in both types of systems may vary considerably at divers times according to water draw off. In many applications, commercial and domestic, and for example for particular appliances such as solar water heating systems, variation in water draw off supply pressure is undesirable and may be detrimental to the operation of the apparatus concerned. It is frequently desirable also to have a low pressure water system when the only available supply source is a high pressure water supply system. For example the present invention may be invaluable in a low pressure solar water supply installation where the head or pressure of an existing supply is often higher than is desirable for efficient operation of the collector for the solar system.